1. Technical Field
The invention relates to processes for the manufacture of foamed articles for refractory, thermal insulation and construction applications such as in the form of bricks, blocks, slabs or discs.
2. Related Art
A problem of a decrease of the density and thermal conductivity is extremely important for producing refractory, thermal isolation and construction articles. This problem is solved by developing new porous materials. An effective method for forming porous structures is the method for the manufacture of foamed materials. There are two main routes presently known for the production of a foamed structure.
I Route:
(a) preparing a stable foam from water and foaming agent;
(b) preparing a wet mixture of solid components, where cement is the main substance;
(c) quick mixing (a) with (b); and
(d) filling the molds with prepared in (c) foam mass.
U.S. Pat. No. 4,158,685 describes the method of making an insulating refractory shape by: (a) preparing a slip of water, deflocculating agent, finely divided solid refractory particles, and binder; (b) preparing a foam of water, air, and foaming agent; (c) admixing the slip and the foam to produce a foamed slurry; (d) casting the slurry into molds; (e) curing; (f) drying the cast pieces so formed; and (g) firing the pieces, the improvement wherein: (1) the slip consists essentially of at least 7.5% cement, from 0 to 50% clay, the balance of the solid ingredients being refractory aggregate all of which passes a 28 mesh screen, all percentages being by weight and based on the total weight of dry solid ingredients, and sufficient water to produce, when mixed with the foam, a foamed slurry having a viscosity of from 1000 to 30,000 centipoises; (2) the foam has a density of not over about 5 pcf, (3) the slip and the foam are admixed in the proportion of from 0.25 volume to 3 volumes of foam for each volume of slip; (4) the cast foamed slurry is cured at a temperature of not over about 72xc2x0 F. for 16 to 24 hours and then (5) dried at a temperature of not over about 200xc2x0 F.
U.S. Pat. No. 3,944,425 describes foamed lightweight ceramic compositions. A fired clay product composed of a rigid cellular mass having a bulk density in the range from about 10 to about 105 pcf obtained by mixing, foaming and firing a composition comprising about 20 to about 95 parts of clay, about 4 to about 35 parts of hydraulic cement, about 0.2 to about 30 parts of inert particulate lamellar foam stabilizer, from 0 up to about 76 parts of inert solid material, about 21 to about 70 parts of water, and a gas generating agent, said parts being by weight of the dry ingredients, all of the foaming action having occurred between cessation of said mixing and about 10 minutes thereafter.
II Route:
A foamed structure is a result of dissociation and/or burning reactions, which go with gassing in heated to the molten state mixture.
U.S. Pat. No. 5,516,351 describes a foamed glass product that can be used as insulation products. The process comprises providing crushed glass particles and a foaming agent, preferably related from CaCO3 or CaSO4. The pretreated glass and foaming agent are sized and mixed. The mixed glass and foaming agent are placed in molds and passed through a furnace where the mixture is heated to a foaming temperature and then cooled to produce foamed glass blocks. Furthermore a non-reactive gas selected from SO3 and CO2 is provided to sweep air away from the mixture during heating. The size of the starting glass particles impacts the insulation properties. A starting glass particle size of approximately 100-700 microns is preferred.
U.S. Pat. No. 3,975,174 relates to a method for manufacture of foamed glass. Finely divided glass, which may have a viscosity between 106 and 107 poises at 950xc2x0 C. to 1100xc2x0 C. and which may be of a composition suitable to conversion into a vitroceramic, is mixed with up to a few percent by weight of a foaming agent including a mixture of SnO2 and SiC, in equimolar or with an excess of SiC. The resulting mixture is heated to 950xc2x0 C. to 1100xc2x0 C. to effect foaming by evolution of CO2 from the foaming agent, and the resulting glass foam is cooled. The nucleation and crystallization steps by which the glass is converted to a vitroceramic may be caused to occur without allowing the foam to return to room temperature.
U.S. Pat. No. 4,992,321 describes a similar method for manufacture of foam glass.
However these methods have some disadvantages.
In I Routexe2x80x94Foamed structure is damaged by mixing solid components with the foam. The time of homogenization is limited and does not allow for the production of a regular, controlled structure of the material.
In II Routexe2x80x94There is an indispensable requirement that the melting and dissociation processes have to coincide at the same temperature interval. This limits chemical composition of the mixtures.
A method according to the invention of forming foamed articles comprises preparing a homogeneous mixture of at least one oxygen-containing, water-insoluble raw material having a particle size  less than 200 microns and at least one gas forming reagent having a particle size  less than 130 microns. The mixture is humidified, shaped and heated to a formation temperature sufficient to melt the raw material to a molten liquid state at which point the raw material reacts with the gas forming reagent to produce a molten liquid foamed material. Once foamed, the material is cooled to a solid state.
The subject method has the advantage of enabling the production of foamed structural articles from different raw materials or combinations of materials for refractory, thermal isolation, construction and other application. The basis of the subject foaming method is a chemical gassing reaction. According to further preferred features of the invention, clusters of carbides and nitrides (boron, silicon, titanium, vanadium, wolfram and etc.) act as universal gas forming reagents. The clusters react with oxygen-containing liquid phase (molten state of the raw materials) at a temperature of its formation. The result of this reaction is gaseous carbon or nitrogen oxides that generate a foamed structure. At heating, the inorganic oxygen-containing natural and synthesized: silicate, aluminasilicate, borate, phosphate in crystalline, vitreous or amorphous state and large number of oxides (B2O3; Al2O3; SiO2; P2O5; ZrO2 and etc.) and/or mixtures of said above raw materials turn into liquid phase (molten state) which, independent of the temperature of its formation, reacts with the gas forming reagent. The speed of the gassing reaction can be advantageously adjusted with great precision by: selection of the particle size of gas forming reagent (carbide, nitride), control of the viscosity of liquid phase (molten state) and duration at the heating temperature.
A method of forming foamed structural articles according to a presently preferred embodiment of the invention includes the preparation of a homogeneous mixture of at least one oxygen-containing, water-insoluble raw material having a particle size  less than 200 microns and at least one gas forming reagent having a particle size  less than 130 microns. The components of the mixture can be sized separately and then blended together or else sized and blended at the same time, depending upon the selection of the components. The mixture is humidified by adding water to the mixture. The amount of water can be varied depending upon the material and the property of the humidified mixture which is desired. For example, the mixture can be humidified to a level where the homogeneous mass exhibits the property of thixotrophy, wherein the material becomes more fluid when agitated such that it can properly and fully fill the shape of a mold cavity and thereafter has the characteristic of setting up and taking the shape of the mold when agitation is discontinued, such that the material can be removed from the mold while retaining the shape imparted by the mold cavity. Such a xe2x80x9cgreenxe2x80x9d molded article may be heated apart from the mold to form the desired foamed structure.
The humidification of the material may be carried out before, during or after the blending of the materials that make up the homogeneous mixture. In other words, the materials may be dry blended and then moisture added or wet blended and moisture removed, if necessary, to achieve the desired humidity level.
The humidified homogeneous mixture is shaped and thereafter heated to a formation temperature sufficient to melt the raw material to provide an oxygen-containing molten liquid phase. The liquid phase reacts, at the formation temperature, with the gas forming reagent to produce a molten liquid foamed material which is then cooled to a solid state. Further details of this basic process are described in further detail below.
The raw materials may be either ductile, inductile, or a combination of ductile and inductile materials. The inductile materials are ground to a particle size  less than 200 microns, preferably  less than 60 microns. The ductile materials do not require grinding since a particle size of clay minerals is considerably  less than 60 microns. Depending on desired properties of the resulting material, single or multiple component mixtures can be prepared from the above-mentioned materials.
Homogenization of the mixture can be reached either by dry or wet combined grinding of the raw materials with fine ground gas-forming reagent or by mixing of separately fine ground raw materials with fine ground gas reagent. As gas forming reagent particles have a size 1.5-3 times as small as particles of ground raw materials, for example  less than 30 microns if the particles of ground raw materials have a particle size of  less than 60 microns, a ratio of particles of gas forming reagent to particles of raw materials in a range between 1:40 to about 1:200 can be obtained. In that case, every particle of gas forming reagent is surrounded by several layers, for example 3-6 layers, of particles raw materials. The fine ground gas forming reagent (carbide or nitride) in the homogeneous mass of single or multiple component mixture achieves the formation of a regular foamed structure.
Formation of articles can be carried out by any of a number of well-known shaping techniques, depending on the selection of raw materials and method of a preparation of the mixture.
A method of vibroformng can be applied to the mixtures containing mostly inductile raw materials at the humidity of the mixtures displaying the property of thixotropy. The required humidity c an be reached by either moisturizing dry mixtures or by pumping the water from a slurry using filterpresses.
To the mixtures of ductile raw materials a s well as mixtures of ductile with inductile raw materials the following methods can be applied:
filterpressing of a slurry,
extrusion of ductile mass prepared by either moisturizing dry mixtures or by pumping the water from a slurry, and
pressing of powders prepared by either moisturizing of dry mixtures or drying a slurry in spray dryers.
The resultant material from the subject method is hardened foam with evenly distributed, isolated pores. The regular distribution of pores improves the properties of the foamed material. The foamed material has an outstanding thermal resistance and strength and is gas and water-proof (i.e., the pores are closed). This material also is low density and displays low thermal conductivity.
The control of the viscosity of liquid phase allows the materials to be heated without the support of any molds made from expensive, heat-resistant stills, since the resulting material keeps its shape even if its volume is increased 4-15 times.
To produce foamed materials from inorganic oxygen-containing natural and synthesized: silicate, aluminasilicate, borate, phosphate in crystalline, vitreous or amorphous state and large number of oxides (e.g., B2O3; Al2O3; SiO2; P2O5) with the temperatures of formation of liquid phase  less than 2100xc2x0 C., SiC (from cluster of carbides or nitrides) may be used because of its relative low cost. For materials having the temperatures of formation of liquid phase higher than 2200xc2x0 C., for example ZrO2; MgO and others, vanadium or wolfram carbides or nitrides can be used as gas forming reagent.
According to the invention, a method for the manufacture of foamed materials from inorganic oxygen-containing natural and synthesized: silicate, aluminasilicate, borate, phosphate in crystalline, vitreous or amorphous state and large number of oxides, is characterized by the following steps:
a) preparing a homogeneous mixture from fine ground raw materials (99.0-99.95%) and fine ground gas forming reagent (0.05-1%) by weight,
b) forming of an article with required dimensions and shape,
c) heating the article at the required temperature parameters, and
d) cooling and calibration of the resulting material.
Silicates include: Talc (Mg3Si4O10(OH)2), Olivin (Mg,Fe)2SiO4), Forsterite (Mg2SiO4), Fayalite (Fe2SiO4), Silicate glass (M2,N) SiXOY where: M=Na, K; N=Ca, Mg, Ba, Fe and others.
Aluminasilicates include: Andalusite (Al2SiO5), Obsidian, Mullite (Al6 Si2O13), Feldspar (M Al)(Al Si)3O8 where M=Na, K, Ca, Fe, Perlite ((Na2O, K2O), (CaO, MgO, FeO), (Al2O3, Fe2O3)nSiO2), Mullitocorund, Aluminasilicate Glass (Na2O, K2O), (CaO, MgO, FeO), (Al2O3, Fe2O3)nSiO2), window glass, bottle glass, other glasses), Kyanite (Al2SiO5), Cordierite (Mg2Al4Si5O18), Nepheline ((Na,K) AlSiO4), Pyrophilite (AlSi2O5(OH)), Sillimanite (Al2SiO5), Vermiculite ((Mg, Fe, Al)3(AlSi)4O10(OH)24H2O), Clays (montmorillonite, illite, hydro mica, kaolinite), Ball clay, diaspore clay, flint clay, kaolin, and burnt clays (chamotte), expanded perlite and other burnt described materials.
Borates include: Danburite (CaB2Si2O8), Borosiliocate glasses, and Datolite (CaBSiO4(OH)).
Phosphates include: Apatite (Ca5(PO4)3OH), Aluminaphosphate (AlPO4), Aluminaphosphosilicate glass, and Aluminachromphosphate.
Oxides include: Quartz (SiO2), Silica (SiO2), Quartz glass (SiO2), Al2O3, Corundum (Al2O3), B2O3, MgO, Chrome ore ((Fe,Mg)(Cr,Al)2O4), Dolomite/Limestone (CaMg(CO3)2), Magnetite ((Fe,Mg)Fe2O4), Rutile (TiO2), TiO2, Spinel (MgAl2O4), Baddeleyte (ZrO2), ZrO2; and others.
Foamed materials can be produced from ductile and inductile materials and their mixtures as mentioned. The plastic or ductile materials include unbumt: clays (montmorillonite, montmorillonitexe2x80x94hydro mica, hydro mica, kaolinitexe2x80x94hydro mica, kaolinite) and kaolin.
The inductile materials are burnt: clays, kaolin and natural volcanic glass (perlite both raw and expanded); oxygen-containing natural and synthesized crystalline, vitreous, amorphous materials, as well as glass-forming boron, aluminum, silicon, phosphorus and other oxides.
Depending on chosen raw materials, the composition of the mixture and the method of its shaping, obtained articles are dried at the different temperature parameters either in special dryers designed for the particular article or materials involved, if need be, or in a furnace directly to remove moisture.
After drying, the articles are heated to the liquid formation temperature. The heating parameters are determined by both composition of the mixture and properties of a resulting material desired.